How to edit my duplicates
I think we're going to need more details and possibly the file, because this is likely something very simple.

1st- how did you create the plane? Was it Add>Plane?
2nd- Did you do anything to the plane before duplicating it, like some modeling?
3rd- At the top right corner, it should let you know how many faces, vertices, etc. there are on the selected object in edit mode. When you get into edit mode how many faces does it say are there?
4th- Did you apply any modifiers and duplicate that with the plane?